1. Field of the Invention
The present invention relates to an image reader and an image forming apparatus using the image reader.
2. Description of the Background
Image readers such as scanners, photocopiers, and multi-functional machines may be constructed to read large-size originals such as AO sheets. AO paper is paper having a height of 1189 mm and a width of 841 mm. These image readers may be implemented using contact image sensors (hereinafter “CIS” or simply “sensors”) to read images of originals transported by transport rollers. In order to reduce cost, the image readers may use a plurality of staggered, partially overlapping sensors to read small-size originals such as A3 and A4, in addition to large-size sheets such as AO.
In image readers equipped with a sensor portion having multiple staggered sensors, the image data read by the multiple sensors must be suitably synthesized to align the image data as if it were read by a single sensor. Such image readers align image data in the main scanning direction by adjusting the reading range of each sensor and in the sub-scanning direction by adjusting the delay time of delay memories provided at each of the sensors based on the arranged distance (gap) between sensors and the transport speed in the sub-scanning direction.
With regard to the delay time adjustment in the sub-scanning direction, a dedicated chart or calibration image on which multiple parallel reference lines in the main scanning direction are recorded along the sub-scanning direction is typically used. The multiple CISs of the sensor portion read the dedicated chart as the original is being transported by transport rollers. Thereafter, the delay time for starting reading images from memories is adjusted by observation of the images where the parallel reference lines are aligned by sight.
However, the actual transport speed of an original is not constant but varies due to eccentricity, deflection, etc., of the transport rollers, non-uniform rotation of the driving motor, friction resistance of parts, etc. As a result, the transport speed changes depending on the position of the dedicated chart in the transport direction. Therefore, a misalignment amount of the parallel reference lines read by the sensors changes so that adjusting the delay time based on the misalignment amount at a single position of the parallel reference lines may conversely result in an increase of misalignment amount at other positions.
However, during transportation of an original, for example, when the front end of the original enters into rollers 13 of FIG. 2 or the rear end of the original passes through rollers 11, physical contact and separation between the original and the rollers may cause vibration referred to as shock jitter.
The original vibrates due to the shock jitter, which leads to changes in the transport speed of the original.
Since the shock jitter suddenly occurs and dies down soon, the transport speed temporarily fluctuates, thereby affecting alignment (synthesis) of images but thereafter the affection dies down.
FIG. 1(a) is a diagram illustrating an original and FIG. 1(b) is a diagram illustrating the result of an aligned image of a read original G with no correction when shock jitter occurs the distance of −1 dot apart between the image sensor arrays. FIG. 1(c) is a diagram illustrating the result of the aligned image of the read original G under the same condition of the aligned image shown in FIG. 1(b) with correction according to Japanese patent application publication no. (hereinafter referred to as JP-A) 2006-109406.
Alignment is not suitably corrected by using the average described in JP-2006-109406-A in the case shown in FIGS. 1(b) and 1(c).